Passive corrosion

Spontaneous Passivation The anodic nose of the first curve describes the primary passive potential Epp and critical anodic current density (the transition from active to passive corrosion), if the initial active/passive transition is 10 lA/cm or less, the alloy will spontaneously passivate in the presence of oxygen or any strong oxidizing agent. 

Passive corrosion caused by chemically inert substances is the same whether the substance is living or dead. The substance acts as an occluding medium, changes heat conduction, and/or influences flow. Concentration cell corrosion, increased corrosion reaction kinetics, and erosion-corrosion can he caused by biological masses whose metabolic processes do not materially influence corrosion processes. Among these masses are slime layers. 

Certainly a thermodynamically stable oxide layer is more likely to generate passivity. However, the existence of the metastable passive state implies that an oxide him may (and in many cases does) still form in solutions in which the oxides are very soluble. This occurs for example, on nickel, aluminium and stainless steel, although the passive corrosion rate in some systems can be quite high. What is required for passivity is the rapid formation of the oxide him and its slow dissolution, or at least the slow dissolution of metal ions through the him. The potential must, of course be high enough for oxide formation to be thermodynamically possible. With these criteria, it is easily understood that a low passive current density requires a low conductivity of ions (but not necessarily of electrons) within the oxide. 

Anodically colored electrochromic inorganic films, 6 579-580 Anodic cleaning, 9 783, 785 Anodic (passivating) corrosion inhibitors, 26 144... 

Final cleaning and passivation shall be performed for removing contamination and corrosion products and to reestablish the passive corrosion resistant surface. 

Solvent UCARSOL Alkanolamine Solution with Metal-Passivating Corrosion Inhibitors... 

In the passive region, values of the passive corrosion rate could be obtained from a polarization curve, though there is no guarantee that it would represent... 

Figure 22 Schematic anodic polarization curve indicating active and passive corrosion regions.

Figure 23 Various reaction scenarios for the passive corrosion of titanium alloys (A) linear oxide film growth due to film recrystallization (B) linear Him growth kinetics maintained by film dissolution.

Based on measurements of this kind, the lifetimes of titanium nuclear waste containers under Canadian waste disposal conditions have been modeled (30). Since these conditions are expected to be anoxic, the passive corrosion of titanium will be driven by a very slow reaction with water, leading to the possibility of hydrogen adsorption into the metal ... 

Since these hydrides are thermodynamically stable in the metal, the passive oxide can only be considered as a transport barrier, not as an absolute barrier. Various electrochemical techniques including EIS and photoelectrochemical measurements have been used to identify the mechanism by which the Ti02 may be rendered permeable to hydrogen, and to identify the conditions under which absorption is observable (31). These determinations show that H absorption into the Ti02 (and hence potentially into the metal) occurs under reducing conditions when redox transformations (Ti1 —> Tim) in the oxide commence. However, the key measurement, if H absorption is to be coupled to passive corrosion, is that of the absorption efficiency. 

Figure 30 The general form of the crevice current (7C), crevice potential (Ec), and planar electrode potential (E ) measured using a galvanic coupling technique. (A) Range of planar electrode (corrosion) potentials (Ef) measured for passive corrosion under oxidizing conditions (B) range of Ef. values measured for passive corrosion under anoxic conditions (C) range of crevice potentials (Ec) measured during active crevice propagation.

Figure 35 Schematic showing the process of hydrogen absorption into titanium under passive corrosion conditions after a period of crevice corrosion.

Localized biological corrosion of stainless steels. There are three general sets of conditions under which localized biological corrosion of austenitic stainless steel occurs (Figure 6.29). These conditions should be examined for metals that show active-passive corrosion behavior. Microbiological corrosion in austenitic steel weldments has been documented. (Wahid)61, (Krysiak)14... 

The second part of the book consists of two chapters namely the forms of corrosion and practical solutions. The chapter, Forms of Corrosion consists of a discussion of corrosion reactions, corrosion media, active and active-passive corrosion behavior, the forms of corrosion, namely, general corrosion, localized corrosion, metallurgically influenced corrosion, microbiologically influenced corrosion, mechanically assisted corrosion and environmentally induced cracking, the types and modes of corrosion, the morphology of corroded materials along with some published literature on corrosion. 

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